Duplicity is a backup program that only backs up the files (and parts of files) that have been modified since the last backup. Built on FLOSS (rsync, GnuPG, tar, and rdiff), it allows efficient, locally encrypted, remote backups.
With the new version of OS X (Leopard) Apple has included some great functionality in Time Machine. Your Mac will automatically backup to an external drive every hour. It includes the ability to recover deleted files in a timeline.
Introduction gDisk is a software that turns your GMail account into a portable hard drive so you can always have your important files accessible accross the Internet.